1. Field of the Invention
The present invention relates to electronic circuits and, more particularly, to signal attenuators that can compensate for signal amplitude changes caused by changes in temperature.
2. Description of the Related Art
A known signal attenuator is disclosed, for example, in Japanese Laid-Open Patent Publication No. 8-125451, which signal attenuator is typically used to compensate for gain variations in a high-frequency circuit (such as a high-frequency amplifier circuit) that includes one or more temperature-sensitive semiconductor devices. Because the gain of such a high-frequency amplifier circuit typically decreases as the ambient temperature rises, the known signal attenuator functions to decrease the signal attenuation as the temperature rises and thereby compensate for gain variations caused by temperature changes. Thus, the high-frequency amplifier allegedly has more uniform characteristics over a variety of temperatures.
FIG. 11 shows a 50xcexa9 xcfx80-type signal attenuator of Japanese Laid-Open Patent Publication No. 8-125451, which includes resistors R41, R42 and R43 and a negative temperature coefficient (NTC) thermistor R44. The resistor R41 and the NTC thermistor R44 are connected in parallel between an input terminal 1h and an output terminal 2h. Input terminal 1h is connected to a pre-stage amplifier circuit (not shown) and input terminal 2h is connected to a post-stage amplifier circuit (not shown).
As a result of experiments performed by the inventors using the known signal attenuator of FIG. 11, the table shown in FIG. 12 was prepared and shows the signal attenuation characteristics of the known signal attenuator. In addition, FIG. 13 is a graph showing the signal attenuation characteristics of the signal attenuator shown in FIG. 11.
In FIG. 12, Rp designates the combined resistance of resistors R41 and R43, Rs designates the resistance of resistor R42 and Rth designates the resistance of the thermistor R44. In FIG. 12 signal attenuation is calculated by dividing the voltage at the output terminal 2h by the voltage at the input terminal 1h. Signal attenuation for four circuits (Examples X1 to X4) having different resistances Rp, Rs, Rth were determined at 25xc2x0 C., xe2x88x9220xc2x0 C. and 60xc2x0 C. and the results are shown in FIG. 12. The signal attenuation of the circuits of Examples X1 to X4 was set to about xe2x88x9220 dB at 25xc2x0 C. (room temperature). The difference between the signal attenuation value (dB) at xe2x88x9220xc2x0 C. and at 25xc2x0 C. and the difference between the signal attenuation at 60xc2x0 C. and at 25xc2x0 C. are shown in the space below each signal attenuation value (dB).
On the other hand, the signal attenuation characteristics shown in FIG. 6, which were prepared based upon a signal attentuator developed by the inventors, have been assumed to be the target or desired values for the known signal attenuator of FIG. 11.
When the resistances of the resistors R41, R42 and R43 and the negative temperature coefficient thermistor R44 are chosen so that the signal attenuation (the median value of the signal attenuation) is set to a specific target value (e.g. xe2x88x9220 dB) at a specific temperature (e.g. 25xc2x0 C.), the variation of the signal attenuation with variations of temperature is uniquely determined. Further, the type of thermister Rth that can be used with this known signal attenuator is limited in practice. Therefore, it is difficult to independently set the desired signal attenuation (the median value of the signal attenuation) for a specific temperature and the desired variation (the gradient) of the signal attenuation as the temperature changes. Thus, useful signal attenuators can not be easily designed using the teachings of Japanese Laid-Open Patent Publication No. 8-125451.
For example, in Examples X1 to X4 shown in FIG. 12, the difference between the signal attenuation at xe2x88x9220xc2x0 C. and at 25xc2x0 C. and the difference between the signal attenuation at 60xc2x0 C. and at 25xc2x0 C. are not the desirable target values shown in FIG. 6. FIG. 13 is a graph showing the signal attenuation characteristics of Examples X1 to X4 and the target signal attenuation characteristics, which characteristics are shown by lines [X1] to [X4] and [R], respectively.
Thus, the signal attenuation (the median value of the signal attenuation) at a specific temperature may be set as the desired target value. However, if the variation (the gradient) in signal attenuation as the temperature changes is not the desired target gradient, the signal attenuator cannot properly compensate for the gain variations of the high frequency circuit caused by temperature variations.
It is, accordingly, an object of the present teachings to provide improved signal attenuators.
In one aspect of the present teachings, signal attenuators are taught that are capable of providing highly useful signal attenuation characteristics over a broad range of temperatures. These signal attenuators can substantially reduce or eliminate variations in signal amplitude caused by temperature-sensitive semiconductor devices.
According to another aspect of the present teachings, signal attenuators may include a first attenuator that does not have a temperature-compensating element and a second attenuator that has a temperature-compensating element. If the signal attenuator only includes the second attenuator having a temperature-compensating element, as described in Japanese Laid-Open Patent Publication No. 8-125451, the signal attenuator will not be capable of providing a desired signal attenuation (the median value of the signal attenuation) at a specific temperature and a useful attenuation gradient as the temperature changes. However, the present teachings provide the first attenuator, which does not have a temperature-compensating element, in addition to the second attenuator that does have a temperature-compensating element. Therefore, the signal attenuator of the present teachings is capable of independently setting the median value of the signal attenuation and the gradient of the signal attenuation. More particularly, the median value of the signal attenuation and the gradient of the signal attenuation can be independently set by combining the signal attenuation characteristics of the second attenuator (i.e. having a temperature-compensating element) with the signal attenuation characteristics of the first attenuator (i.e. not having a temperature-compensating element).
Preferably, the first and the second attenuators may comprise resistors and a thermistor may be the temperature-compensating element. With this construction, the signal attenuation characteristics of the signal attenuator do not change with changes in signal frequency. Therefore, this signal attenuator may preferably be utilized to attenuate signals from a temperature-sensitive high frequency circuit, such as a high-frequency amplifier.
Further, the first and the second attenuators may be preferably connected in series. With this construction, the signal-attenuation of the signal attenuator is determined by the sum of the signal attenuation of the first attenuator and the signal attenuation of the second attenuator. Therefore, the signal attenuation characteristics of the signal attenuator can be easily set.
Further, at least one of the resistors in the first attenuator may preferably be a variable resistor. In this case, the signal attenuation characteristics of the first attenuator can be adjusted by adjusting the resistance of the variable resistor of the first attenuator. Therefore, the signal attenuation characteristics of the signal attenuator can be easily set. Such signal attenuators may be preferably utilized to encode data in a radio transmitter that uses spread spectrum signals.
Additional objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims.